Dental materials include porcelain facings, veneers, bridges, inlays, crowns, and a multitude of other products. The first step of the casting of, for example, an inlay or a crown, is the preparation of a wax pattern. The cavity is prepared in the tooth and the pattern is carved, either directly in the tooth or on a die that is a reproduction of the tooth and the prepared cavity. If the pattern is made in the tooth itself, it is said to be prepared by the direct technique. If it is prepared on a die, the procedure is called the indirect technique. However the pattern is prepared, it should be an accurate reproduction of the missing tooth structure. The wax pattern forms the outline of the mold into which the alloy or ceramic is cast. Consequently, the casting can be no more accurate than the wax pattern, regardless of the care observed in subsequent procedures. Therefore, the pattern should be well adapted to the prepared cavity, properly carved, and the distortion minimized. After the pattern is removed from the cavity, it is surrounded by a material which forms an investment. This process is called investing the pattern.
Commonly used investment materials include gypsum, phosphate and silica-based materials. Preferably, silica-based investments fabricated from all or a high percentage of quartz or cristobalite are used as dental investment materials. After the investment material has hardened, the wax is eliminated, typically by heat to provide a mold cavity for forming the dental restoration. The investment includes a pouring channel which is formed by a sprue on the wax model. This provides a channel through which the dental materials are supplied to the mold cavity. Dental materials, such as dental ceramics, may be inserted into a premolding space in the form of an unfinished piece or blank. The blank is softened by heat so that it can be introduced into the molding cavity in a viscous state using fairly low pressure to assume the shape of the mold cavity to form the desired dental prosthesis. This process is called heat pressing and is described in "Hot-Compressed Porcelain Process For Ceramo-Metal Restorations" by E. R. McPhee in Dental Porcelain: The State of the Art--1977, edited by Henry Yamada, USC School of Dentistry, Los Angeles, Calif. More recently, the process was described in an article by M. J. Cattel et al., entitled "The Biaxial Flexural Strength of Two Pressable Ceramic Systems" in Journal of Dentistry 27 (1999) 183-196.
A pressing furnace is used to press the ceramic material into the mold cavity and conform the material to the shape of the cavity. The pressing furnace includes a driving plunger (herein referred to as the "internal plunger") that contacts a second or external plunger inserted into a cylindrical mold made from refractory investment. This cylindrical mold is known as an investment ring in the dental field. The external plunger is in contact with the ceramic material. The external plunger transmits the pressing force from the driving plunger to the ceramic material and forces the material through the channel to the cavity. After the pressing operation, the internal plunger is raised, the investment ring and the external plunger are removed from the pressing furnace, and the resultant dental material is removed from the mold. The external plunger must be cleaned after the pressing operation.
In current practice, alumina plungers are used as external plungers. The alumina plungers are intended to be reused and are relatively expensive. Since the external plunger is in direct contact with the ceramic material, it may react or adhere to the ceramic material. Consequently, cracks may form, originating at the interface between the plunger and the porcelain button, and propagating through the pressed shape. One reason for this is the variation in shrinkage and cooling rates between the plunger and the ceramic materials used to form the dental restoration. Moreover, if the ceramic materials stick to the plunger, cleaning becomes difficult, rendering reuse inconvenient and problematic. Removal of the glass-ceramic from the plunger requires grinding, sandblasting, and/or soaking the plunger in acid resulting in usage of time and labor which could otherwise be spent on other more constructive tasks. Furthermore, this cleaning process can weaken and distort the plunger reducing the useful life of the plunger. Other techniques have been attempted to solve plunger-related problems, such as modification of the pressing cycle or usage of a massive metal block as a heat sink to promote fast cooling of the ring in an effort to avoid cracking. Nevertheless, these solutions have not been proven to be fully effective and cracking may still occur from time to time.
There is a need to reduce or eliminate time involved in cleaning the external plunger after completion of the pressing operation. It is desirable that cracking problems occurring during the pressing operation be reduced or eliminated.